Great attention has been paid to ceramic materials for their heat resistance, wear resistance, high-temperature strength and other advantages. However, ceramic materials are extremely difficult to mechanically work since they are hard and brittle. Thus most ceramic articles are prepared by sintering and precursor methods. The sintering method involves the step of pressing or otherwise molding a ceramic material in powder form into a desired shape followed by sintering. The precursor method is by melting an organic polymer as a ceramic precursor or dissolving it in a suitable solvent, molding the melt or solution into a desired shape, and then sintering for converting the polymer into inorganic form. The precursor method is characterized by the potential manufacture of ceramic articles to a configuration which cannot be achieved with the powder sintering method, and especially adapted for the manufacture of fibers.
Among ceramics, SiC and Si.sub.3 N.sub.4 are of great interest for high-temperature performance, more particularly because of heat resistance and high-temperature strength for the former and thermal shock resistance and fracture toughness for the latter. Extensive research works have been made on their precursors. The silicon carbide and nitride ceramic materials are also considered useful as reinforcements for fiber-reinforced composite materials by taking into account their light weight, heat resistance, and high strength features. Thus integration of these ceramics with plastics, metals and other ceramics is also an important subject.
In the prior art, ceramic fibers are prepared by forming an organic silazane polymer through pyrolytic polymerization and converting the polymer as a precursor into ceramic fibers composed of SiC and Si.sub.3 N.sub.4 as described in U.S. Pat. No. 3,853,567 (Japanese Patent Publication No. 46995/1980). This method produces an organic silazane polymer by heating a silazane compound resulting from a methylchlorosilane and an amine to a temperature in the range of from 200.degree. to 800.degree. C. and polymerizing the compound in a Raschig ring packed column. This method has the following problems.
(1) Only limited reactants are available. The patent specification sets forth only methyltrichlorosilane and dimethyldichlorosilane as the methylchlorosilane and monomethylamine as the amine.
(2) This method produces an organic silazane polymer by passing a monomer or silazane compound through a column loaded with packings such as Raschig rings. As will be evident from Comparative Example described later, this method allows the monomer to polymerize upon contact with the packings, but the extended contact with the packings can cause some polymers to convert into higher polymers. Such insoluble, infusible, highly polymerized solids will gradually accumulate in the column, disturbing further continuation of polymerization reaction.
(3) The method based on a Raschig ring packed column allows a considerable amount of a crystalline by-product of the structure represented by formula (1) below to form. The by-product precipitates and deposits on the gas phase-defining interior wall of the reactor, participating in reaction no longer. As a result, the end organic silazane polymer is obtained in a low yield of 36%. ##STR1##
(4) The resulting organic silazane polymer is less resistant against hydrolysis.
For these drawbacks, the method of U.S. Pat. No. 3,853,576 is difficult to commercially effectively prepare an organic silazane polymer which is a ceramic precursor.
Most often, ceramic fibers are prepared by spinning an organic silazane polymer, infusibilizing and sintering the polymer to render it inorganic. The above-cited U.S. Pat's method has the following problems in the fiber-forming procedure.
(1) The organic silazane polymer tends to increase its melting point in a spinning bath which is formed by heating and melting the polymer. The melt is thermally less stable and less spinnable so that frequent breakage occurs during spinning.
(2) Since the organic silazane polymer is less resistant against hydrolysis, it is likely to take in oxygen during fiber formation so that there are obtained fibers of poor quality.
(3) The resulting ceramic fibers have low strength.
(4) Ceramic retention is low.
Therefore, the above-mentioned method is not adapted for commercially effective manufacture of inorganic fibers of quality consisting essentially of Si, C, and N. It is thus desired to overcome these drawbacks.
Further, since the above-mentioned method produces ceramic fibers composed mainly of SiC-Si.sub.3 N.sub.4, it is not applicable to the preparation of silicon nitride fibers, that is, fibers composed mainly of Si and N.
Many methods are known in the art for the preparation of ceramic fibers of silicon nitride, for example, Japanese Patent Application Kokai (JP-A) No. 135431/1985 (U.S. Pat. Nos. 4,540,803 and 4,543,344), JP-A 125015/1987, JP-A 12915/1986 (U.S. Pat. No. 4,650,773), and JP-A 232270/1986 (U.S. Pat. No. 4,761,389). The methods of JP-A 135431/1985 and 125015/1987 suffer from a problem associated with organic silazane polymer precursors being less resistant against hydrolysis. It is inevitable for oxygen to incorporate into ceramic fibers during fiber formation so that the fibers have an uncontrolled content of oxygen and become deteriorated in strength and heat resistance. JP-A 12915/1986 or U.S. Pat. No. 4,650,773 discloses a method for preparing silicon nitride fibers in an atmosphere of ammonia gas. Instead of the organic silazane polymer, this method uses a polycarbosilane precursor which is commercially produced in low yields and thus expensive and uneconomical. Further, JP-A 232270/1986 or U.S. Pat. No. 4,761,389 discloses a method for sintering a polycarbosilane or polysilazane in an ammonia-containing atmosphere. The object of this method is to produce a ceramic material having a reduced carbon content. Although a ceramic material approaching the stoichiometric composition of silicon nitride (Si 60% and N 40%) can be produced by minimizing the carbon content, the resulting ceramic material has a substantially reduced nitrogen content and a markedly reduced strength. This suggests the presence of a substantial amount of oxygen, indicating a potential lack of quality.
Therefore, the above-mentioned methods are not adapted for commercially efficient manufacture of inorganic fibers of quality consisting essentially of Si and N. It is thus desired to overcome these drawbacks.